Wire providing microwave/radar frequency attenuation is referred to in the wire and cable trade as "filter line". Some of these cables are referenced by U.S. Mil. Spec. No. Mil-C-85485. The measurement of the attenuation (insertion loss) upon a given wire's performance relates to the effect that the filter line has upon the interference signals which are conducted down the wire. Such wire and cable can also be shielded from external EMI signals by providing an overlayer of metallicized, braided or served mesh.
The ever-increasing high technology requirements of the aerospace industry demand that filter line and shielded cables be more flexible and lighter in weight. Utilizing the commercial materials that are presently available, some of the latest industry requirements cannot even be met.
This invention seeks to provide new, space-age materials in unique combinations to provide EMI, microwave and radar filtering and shielding capabilities in a wider frequency range than heretofore available. At the same time, the invention provides a cable product that utilizes thinner filtering and shielding layers, which reduces the size of the composite cable construction. In addition, the newer materials are lighter; their thinner cross-sections further reduce the weight of the finished cable product. The invention utilizes a shielding of metal-coated, high-tensile strength fibers (such as nylon or aramid) which greatly improve the flexibility and strength of the overall composite construction.
Recently, ferromagnetic particles of ferrite or magnetite have been coated with metal in order to provide conductive materials having advantageous electrical and magnetic properties. It is contemplated with this invention that these types of materials can be loaded into a polymeric matrix for use as an interference layer in the fabrication of "filter line" cable.
The filter line cable of this invention can be further shielded for outside interference signals over an extended frequency range; the shielding layer of the invention provides a layer of metallized fiber braiding, as taught by the aforementioned, parent patent application, Ser. No. 07/862,871. The metallicized fibers are woven or served into a mesh that is layered over the "filter line" attenuating layer with an approximately 95% coverage.
Properly shielded filter line provides protection against radiated EMI. Noise currents and voltages are induced on the conductors of the cables when a radiated field causes interference. Filter line can attenuate such noise when it is shielded with a metallic braid or other forms of shield layering. The shielding effect can be measured by transfer impedance techniques.
The present invention seeks to fabricate wire and cable articles that provide protection against both of the aforementioned effects, namely, attenuation of signals conducted down the wire and radiated interferences penetrating the cable.
The current invention contemplates a wire or cable construction using a layer composed of magnetic particles such as ferrites or magnetites dispersed in a polymeric matrix, such as Viton (a fluorinated elastomeric polymer manufactured by Du Pont). The magnetic particles are provided by various manufacturers, including Steward Manufacturing Company of Tennessee and Fair-Rite Products Corporation of New York. The impedance characteristics of the magnetic particles vary, depending upon the supplier, fabrication conditions and composition. Metal coating (such as silver) is provided by Potters Industries, Inc., of Parsippany, N.J.
High-frequency signals conducted down this wire are partially absorbed by the magnetic particle shield layer. The electromagnetic waves penetrate through this shield layer up to the ferrite particles and are then dissipated by lattice vibration or phonon emission. Improved protection against external interference is provided by the shield layer, as a more effective mesh layer is provided by tightly-woven or served flexible fibers. The metallized surface area comprises a silver coating on high-tensile strength fibers, made up of materials such as nylon, aramid, etc. The advantages of such a construction include cost savings, streamlined economy of size and weight, and improved flexibility and tensile strength, as compared to the conventionally shielded cable, made up of traditional metal wire mesh shields. The metal-coated fibers provide an approximately 95% coverage or better. Streamlined weight, improved flexibility and tensile strength are particularly important in view of the stringent requirements for present-day, light-weight aeronautical wire and cable.